chore(readme): optimize the text description in README

[pkudba]

Description

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [Docs], [Fix], [Feature], [Refactor])
• Related issue is referred in this PR
• The markdown and latex are rendered correctly.
• The code in PR is well-documented.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR.